In order to monitor the state of the reactor core, in particular for the purpose of raising damaged sheaths to show up faults and to monitor changes therein, it has been the practice to use gamma spectrometry to analyze the various isotopes (of xenon, of krypton, of iodine) present in samples taken from the primary water circuit, with said samples being taken manually and individually.
Unlike the technique of detecting delayed neutrons as emitted by certain short-life isotopes, which technique is easily automated and also gives information about the integrity of the fuel, gamma spectrometry serves to show up faults which are at extremely low density and to provide highly accurate information about the state of the sheaths, thereby making it possible to specify, during operation, the number of faulty fuel rods and the degree to which their sheaths are damaged.
The frequency with which gamma activity is measured may vary from one or two samples per day or per week when the power station is operating under steady conditions, to one sample every 15 minutes to 3 hours during transient stages in power station operation if it is desired to track the bursts of activity that result therefrom.
Analysis by periodically taking samples, the sole technique which has been used in the past, gives rise to numerous difficulties: in addition to the tedious and expensive nature thereof (in particular because people are required to take the samples and to handle analysis equipment), the most significant drawback lies in the impossibility of having analysis results available immediately, firstly because of the time lost in taking samples and in transporting them, and secondly because of the time required for analyzing the samples and interpreting the results.
One of the objects of the present invention is to provide a measurement device enabling such gamma spectrometry of the cooling fluid to be performed in real time in a manner which is fully automated.
The major difficulty in providing an on-line device is avoiding the disturbing effects of corrosion products and of solid fission products depositing on the walls of the sampling volume.
Instead of the weekly or bi-weekly manual sampling operations currently recommended, the automatic device makes it possible to obtain at least two measurements per day without difficulty, and there is no additional constraint in recording the bursts of activity that follow operating transients, merely by triggering high frequency measurement sequences during said transient stages.
In addition, the possibility offered by the on-line gamma spectrometry technique of taking measurements at a high frequency makes it possible to seek correlations between the appearance of breaks and the operating parameters of the reactor, which are themselves acquired by the automatic device.
It is also shown that by virtue of the various characteristics of a device in accordance with the present invention, it is possible to obtain measurements which are extremely reliable, which do not have uncertainties due to decay periods or to metering, and which therefore make it possible to follow the state of the reactor core very accurately, without being hindered by corrosion products or solid fission products.